Method and apparatus for recovering a light metal



R. J. MCNITT Aug. 14, 1945.

METHOD AND APPARATUS FOR RECOVERING LIGHT METALS Filed Oct, 15, 1941 4 Sheets-Shears?I l Z (bref .6 N A ,fR

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METHOD AND APPARATUS FOR RECOVERING LIGHT METALS Filed 001;. 13, 1941 4 Sheets-Sheet 2 J/ aff/afa] ATTORNEYS R. J. MCNITT Aug. 14, 1945.

METHOD AND APPARATUS FOR RECOVERING LIGHT METALS Filed Oct. 13, 1941 4 Shees-Sheet 5 i BY 7,3m, 0mg/www #Mn/m14 ATTORNEYS R. J. McNlTT 2,382,434

METHOD AND APPARATUS FOR RECOVERING LIGHT METALS Aug. 14, 1945.

Filed Oct. 13, 1941 4 Sheets-Sheet 4 Patented Aug.. 14, i945 METHGD AND APPARATUS FOR RECOVR- ING A LIGHT METAL Robert J. McNitt, Perth Amboy, N. J.

Application October 13, 1941, Serial No. 414,752

si claims. (o1. 'z5- 93) impurities, and with less trouble and expense due to blockages in the apparatus.

The crude metal as collected in the bath may contain such impurities as salts from the bath, other compounds of the metal sought, carbon, and other metals and their compounds. Some of these impurities are merely entrapped asparticles in the crude metal. Other impurities are dissolved in the metal at the temperature of the fused bath but may be more or less completely thrown out of solution at lower temperatures. Such of these impurities as can be made to settle out of the crude metal by gravity are designated herein as sedimentary impurities. Hereinafter the fused bath from which the light metal is recovered will be called the bath and it is understood that the metal is in a molten or liquid state.

In the recovery of light metals from fused baths it is customary to collect globules of the metal in the bath. To facilitate the removal of the metal from the bath it has been found advantageous to confine this metal Within a duct through which the metal rises above the upper surface of the bath and overows into a receiver, while the sedimentary impurities fall back into the bath. Although widely used for many years this method has not given uniformly satisfactory results. Particles of the bath which are carried into the duct with thelight metal congeal as the temperature of the rising metal falls due to the ilow of heat to the surrounding air above the surface of the bath, and the freezing particles of salt form with the metal and other impurities cement-like accretions on the walls of the duct` which must be removed by Scrapers to prevent the obstruction from completely blocking the upward flow of metal. Even with the use of Scrapers and agitators, blockages occur which seriously restrict the ow of metal and make necessary the replacement of the duct. This difficulty not only causes considerable expense but, through the operation of the scraper, it interferes with the settling back of the sedimentary impurities.

Another source of interference with sedimentation lies in the gases which are liberated in the fused bath and collected with the crude metal. These gases such as hydrogen, carbon dioxide and carbon monoxide enter the duct with the metal and rise through the column in bubbles which not only interfere with the return of sedimentary impurities to the bath but sometimes carry slugs of crude metal and masses of impurities over into the receiver.

These diiiiculties are aggravated when the crude metal contains impurities which are held in solution at the temperature of the bath but are precipitated when the temperature falls as the metal y rises above the surface of the bath, for notonly is the tendency towards the formation of accretions and blockages greater but, due to the greater temperature dierence between the upper and lower parts of the duct, convection currents in the metal are stronger causing greater interference with sedimentation. Furthermore, some of the precipitated particles are likely to be very viinely divided and even though they be much more dense than the metal, they fail to settle rapidly enough to avoid being carried upward by the rising metal. It is the aim of my invention to overcome these diiiculties and to remove these impurities more eiiectively from the metal.

In accordance with my invention I collect or accumulate the crude metal below the upper surface of the bath beneath a hood in any suitable manner. The crude metal may be in the form of globules, coalesced particles or thin lms. It may also be in the form of layers of varying thickness. I may advantageously accumulate metal in a suiiiciently deep layer to form a pool, and control the movement of metal in the pool to favor the separation of sedimentary impurities before removing the metal from the pool.

When the metal is collected in a deep layer or pool, I prefer to provide such an area of metal, and to control the i'loW of metal and convection currents therein to maintain a large part of the metal in such a quiet state that sedimentaryimpurities such as entrapped particles of salt fall back into the bath. Gas which is collected with the crude metal interferes less seriously with the Separation of sedimentary impurities if it is collected separately and prevented from indiscriminate bubbling through the pool of metal.

The metal collected under the hood is passed upward to a body of metal above the bath Where conditions are controlled to facilitate a further settling of impurities and the precipitation of dis- Solved impurities. 'The body of metal is, of course,

connected with the'bath by means for receiving metal from the bath, and also by other means for returning the impurities to the bath, but is otherany suitable confining means such as a duct and the temperature of the duct or the rate of ow of metal therein is controlled to suppress the foration of accretions on the duct. When the metal is subjected to a preliminary purincation'beneath the bath, there is less tendency toward the for-f mation of lobstructions in the duct which carries the metal up out of the bath and a better opportunity for a, more complete separation of the remaining impurities.

I prefer to collect gas in a layer above the of metal and may liberate it by letting it bubble through the rising stream `oi* metal, or I may provide special ducts for that purpose. The gas is so removed that its upward movement does not impede the separation and settling of impurities.l I prefer to pass the hot crude metal of the rising stream along th'e upper portion of the body of metal, maintaining the temperature of the latter below the temperature of the crude metal admitted, thus avoiding agitating currents in the body of metal. I prefer also to pass' the hot metal from the rising stream along the upper surface of the metal in the body and to effect such a cooling of the hot crude metal that dissolved impurities are precipitated and fall into th'e body of metal from which they are returned to the bath through a column of metal connecting the body of metal with the bath. In an important embodiment of my invention, I suppress the flow of heat from the bath, or from the metal in th'e b ath, through the column of metal in which the impurities are returned. to the bath to minimize thermal convection currents in the body of metal.

In accordance with my invention, I connect the chamber for confining a body of metal above the bath with a duct which leads into the bath and through which impurities may settle to the bath.' I prefer to use a duct, having the capacity to suppress the iiow of heat from th'e bath, or the metal below the bath, into the metal in the chamber and thereby maintain the metal in the chamber, 'and also in parts of the duct, in such a quiet state that it is free of convection currents vwhich,

interfere with the settlingof impurities and their return to the bath. I may accomplish theseresults in asimple manner by so constricting the cross-sectional area of the return duct that it oii'ers high resistance to the ilow of heat. In other applications of the invention I insert a thermal insulation in the duct to suppress the upward now of h'eat, remove heat from the metal in the duct before it reaches the body of metal, or place between the bath and the retum duct heat control means, such as insulation or a cirlayer Y erably comprising metal collecting means in the bath in which layers of crude metal and gas may collect and a plurality offconduits or ducts connecting the collecting means with a chamber positioned above the upper surface of the bath for confining vthe body of metal. 'I'he means for the collection of metal may be the usual hood in the usual hood and mount a removable cover culating fluid, to prevent an excessive exchange the hood cover to pass the stream ofcrude metal and gas upward into the chamber. I provide at 'least one return duct connecting the chamber with the bath through which impurities are returned to the bath. I may insert this duct in the bath in any suitable place, for example, outside the means for collecting the metal. I may connect the return duct or ducts to the hood or pocket and return the impurities directly under the hood. The return duct, or ducts as the case may be, is either so constructed, orso provided with means, that the ilow of heat or gas therethrough is suppressed and convection currents in th'e duct and in the body of metal in the chamber are either eliminated or so eii'ectively minimized that they do not interfere with the settling of sedimentary impurities and their return to the bath.

The chamber for confining the body of metal may be of any suitable size or shape but is preferably in the form of a hopper and it may comprise a single chamber or a plurality of connected receptacles, each' preferably connected at the lower portion to the bath by a separate return duct. The temperature of the chamber is controlled to cool the body of metal suiliciently to precipitate the dissolved impurities, and when I use a chamber having a number of connected receptacles, I may progressively decrease the temperature of the metal from receptacle to receptaclein the direction of metal ilow to effect a more complete purication of the metal. I so control the temperature of the chamber walls, however, as to prevent the formation of accretions thereon. Especially whenvI pass the metal from receptacle to receptacle and cool the metal to just above its fusion point, I may heat the upper surface of the metal before it ows into the receiver to decrease its viscosity and facilitate the settling of impurities.

In an effective construction according to the invention, the chamber, together with parts of the ducts above the bath, or above the cell cover,v

is covered ,with a housing through which air is circulated to control the temperature.- I prefer to provide baille means in the chamber t0 control the ilow of metal therein so that the sedimentary impurities will be directed downwardly.

Reference has been made herein to a column of metal, and also to return ducts, connecting to v or extendinginto the bath, and it is to be understood that this applies to the bath proper or to the layer of metal collected under the bath because ultimately the impurities settle into the bath even when they are returned only to the layer ofmetal.

The accompanying drawings illustrate apparatus embodying the invention, in which:

Fig. 1 is a plan view of one form of electrolytic cell with the cover removed; i

Fig. 2 is a sectional elevational view along the line 2-2 of Fig. 1;

Fig. 3 is a horizontal sectional view from below and along the line 3--3 of Fig. 2;

Fig. 4 is the development of a. vertical sectional view, with parts removed, along the lin d-l of Fig. 1;

Fig. 5 is a vertical sectional view of a modied form of the apparatus taken at a place similar to that of Fig. 4;

Fig. 6 is a cross-sectional view of a baille taken along the line @-6 of Fig. 4;

Fig. 7 is a vertical sectional view of a modified form of the apparatus taken at a place similar tb that of Fig. d;

Figs. 8, 9, l0, 1l, l2 and 13 are cross-sectional views of other formsgof apparatus taken at places similar to that of Fig. 4, and

Fig. 14 is a sectional elevational view of another apparatus for recovering metal.

As an example, I shall describemy invention as applied to the recovery of sodium from a fused mixed bath of sodium and calcium chlorides, such as is generally used in the electrolytic production of sodium.

Figs. 1, 2 and 3 illustrate a form of electrolytlc cell in which the improvement of the invention may advantageously be applied.` It comprises a metal shell, a refractory lining, a carbon anode, a metal cathode, a diaphragm, for example a. screen, arranged between the electrodes, a gas collector above the anode, a hood for the collection of metal, and a removable cell cover. During the normal operation the upper surface of the bath is at the place indicated.

The apparatus shown in Figs. 1 to 4 comprises a pocket I for preliminary puriiication of metal, and an upright duct 2 connecting the pocket 'with Va chamber 3 above the bath. The chamber is constructed to receive metal from the duct 2, and while it may comprise only 9, single receptacle for conning a body metal, it may, as shown, comprise several connected receptacles Il and 5 having downwardly sloping hopper-shaped bottoms the lower ends of which are connected by return ducts 6 and I, respectively, with the bath in the pocket. The chamber 3, and particularly the receptacle 5 thereof, is connected by conduit 8 with a receiver 9 for receiving puried metal from the vessel 5. The hood has upwardly extending walls It over which the cover II is placed forming the pocket I. A

The ducts 2, and 1 project through the top of the cover II and. are preferably welded thereto. The lower ends of the return ducts 6 and I have flared or enlarged portions I2 and I3, respectively, to avoid accumulations which tend to form at the plane of separation between the layer of metal and the fused bath. It will be seen that the duct 2 extends slightly into the pocket and the lower edge thereof establishes the thickness of the layer of gas and the upper surface of the layer of metal. are free to enter duct 2 and now into the chamber 3.

In order to prevent chilling and suppress the formation of accretions in duct 2, the upper exterior portion thereof (above the cell cover) is provided with thermal insulation I5. If the rate of metal production is high and the duct 2 is of small diameter, there is practically no tendency for thisduct to become blocked with incrustations due to cooling of the metal and the insulation is unnecessary. If the rate of production is relatively low, I prefer to place the thermal insulation around that part of duct 2 which stands above the cellcover. I may apply electric heating resistors to the duct if additional heat is necessary to Vprevent undue cooling, or I may make a small opening I6 in the wall of return duct 6 within the layer of sodium in pocket I to permit the dreturn of some of the cooler and denser metal from the receptacle d to the pocket I, thus increasing the rate of ow of metal upcrude hot metal to flow horizontally through.

the chamber without appreciable agitation. Gases rising with the crude metal in duct 2 pass through the space above the metal and are released through valves I 'I or I8. Capped openings I9 are arranged on the top of the chamber to permit inspection of the chamber and the cleaning of the receptacles 4 'and 5 together with their attached ducts 6 and l.

A removable sheet metal housing 20 is placed around the chamber 3 and receiver 9 and warm air is circulated around the walls of the chamber at such a rate that the temperature of the metal overflowing in the conduit 8 may be decreased to a few degrees above its melting point. A damper controlled chimney 2| is connected to the housing and by controlling the natural draft, a satisfactory control of the temperature may be accomplished for moderate rates of production. When the rate of metal production is high, adequate circulation of the air may be maintained by means of a small blower. The air may be preheated by drawing the air under the bottom edge of the housing and over the cell cover, or in any other manner.

In order to counteract the effect of slugs of metal carried up duct 2 with the gas and to aid in the separation of impurities fromtl'iev metal, I may arrange one or more bailies 24 across the vessels. The baille will4 change the direction of flow of the metal, as shown by the arrows, and

` direct the impurities towards the ductieading The metal and gas back into the pocket. As shown in Fig.,6, the baille 24 is in the form of a trough which directs the suspended impurities into the bottom opening of the receptacle. The purified metal may now through the passages 25 between the'trough and the side walls of the chamberv 3, thus causing less interference with the separation of impurities.

The hood has a depending annular wall 3U extending into the bath. The skirt or baille 3| is attached at 32 to the wall 30 and extends around the opening into the pocket terminating at 33.

the flow of heat through the metal itself. To this end ducts B and 1 of Figs. 1 to 4 are made as small as practicable in view of returning the imv purities to the bath continuously in the absence of obstructions in theY duct. The cooling chambers 35 and 36 are placed above the cell cover and afford considerable cooling surface. The metal in these lcooling chambers is chilled and this not only dampens out thermal convection currents flowing between the metal in the ducts and the metal in chamber 3 above. but by reducing the temperature in the upper ends oi' theducts, the now of heat from the ducts into the lower part of receptacles 4 and I is suppressed. This form of the invention involves a dissipation of heat from the metal in the duct to suppress the receptacle 4 and into and across receptacle siderably lower than the temperature of the hot,

'I'he hot crude metal from duct 2 flows across l and overflows into the conduit 3 from which it enters the receiver I. The body of metal in chamber 3 is maintained at a temperature concrude metal overflowing from duct 2. As will be explained more fully hereinafter, this temperature is controlled by dissipating heat from the chamber. In flowing across the body of cooler metal in the receptacles 4 and 5,' the temperature of the incoming crude metal is reduced to within .a few degrees of the melting point. Impurities which were heldin solution at the higher temperatures are precipitated and fall with other sedimentary impurities into the underlying body of the ilow of heat upward therefrom into the body l of metal in the chamber.

I may diminishthe amount of heat which must be dissipated in this manner by placing a thermal insulation between the return ducts and the bath, and I may also restrict the cross-sectional area of at least portions of the return ducts below the cooling chambers 35 and 36 to oder greater resistance to the upward ilow of heat as described in connection with Fig. 8. Y

In certain operations, the gas rising through the bath may enter the return ducts rising therekin and agitating the metal in the chamber. This condition may be particularly aggravated when the return ducts have flared lower ends. To eliminate this condition, I mount bailles 31 in the bath directly beneath the return ducts.

' In carrying out a method of the invention in an electrolytlc cell embodying the improvements illustrated in Figs. l to 4, the molten metal rising between the diaphragm and cathode passes up under the hood where it is trapped within the depending wall 30. The metal is prevented from entering the pocket I along the baille 3| but is free to enter the pocket through the opening 34 and pass upward therein. -I prefer to 'so proportion the distance between the level of metal in chamber 3 and the upper surface of the bath and the distance between the lower surface of the metal in the pocket and the upper surface of the bath as to keep a considerable depth of metal in the pocket I, for example, alayer from 2 to 8` inches thick, depending upon the amount of salt particles in the metal which vary with the amount of gas collected with the metal. The metal flows across the top of the layer from the left end of the pool to the duct 2 in a gentle stream and the gas passes to the top of the pocket without agitating the metal in the pocket. These conditions facilitate the removal of particles of bath and other sedimentary impurities from the metal before ii rises in the duct 2. As the metal accumulates in the pocket I, it is forced upwardly by reason of its low specific gravity through the duct 2 from which it overflows into the receptacle 4 of chammetal. Striking the hopper-shaped bottom, these impurities slide into'ducts 8 and 1 and settle therethrough into the bath. With this meth'- od of separation there is little tendency for masses 'of impurities to collect at the walls of the separating vessels in they form of cement-like incrustations. By providing receptacles 4 and 3 with ample external surface, a cover, and by circulating warm air around them instead of exposing the bare sides of the vessels to the cold drafts of the room, I reduce the tendency for incrustation on the internal surfaces of the walls of these receptacles. Likewise by thermal insulation, the application of electric heat and a rapid flow of the metal therein, I maintain the walls of the upper part of duct 2 hot enough to avoid the formation of blockages in this duct, thus avoiding the frequent use of Scrapers and agitators therein. Y

The relatively low temperature of the metal in the lower part of the receptacles and its quiescent state is due in part to the substantial absence v of currents due to heat (and also gas) flowing up- "0 ward through the return ducts to the chamber.

ber 3. The gases also pass upwardly through duct I 2 and escape from chamber 3 through valves I1 or I8 which are adjusted to allow a slight pressure in the chamber. thereby preventing the infiltration of air.

As explained above, this flow of heat is reduced to a minimum and this facilitates the cooling of the metal in the body confined in the chamber 3 because there is much less heat to dissipate than if heat were permitted to flow without restriction into the body of metal. The `temperature of the metal in receptacle l may be somewhat lower than the temperature of the metal in receptacle 4 under certain circumstances as, for example, when metal is permitted to circulate through opening I6 in the lower part of duct B. On the other hand, when the amount of metal recovered is great, I may make the temperature of the metal near the overflow to the receiver higher so that the metal flows into the receiver well above the melting point (l50 C.) thus reducing the viscosity of the metal' and facilitating the downwardflow of impurities so that the metal entering the receiver may be used for practically all purposes without further purification.

Theapparatus of Fig. 5 is similar to that of Figs. L to 4, with the exception that the gas is removed from the pocket through a separate duct 40 which connects to an extension 4I directly above the place where the metal and gas enter the pocket. 'Ihe upper part of duct 40 is enlarged to facilitate a separation of gas from the metal and has thermal insulation above the cover. 'Ihe baille 43 directs the gas and metal to the top of the pocket.

When the apparatus of Fig. 5 is used in carrying out a method of the invention, the gases are removed from the hood in the duct 40, thereby eliminating the possibility of slugs of crude metal being hurled into vessels 4 and 5 and interfering In order to aid in controlling the quantity of with the separation of impurities therein. In view of the manner in which the duct 48 connects to the pocket, there is no layer of gas accumulated under the pocket. 'Ihe gas escapes to the atmosphere through valve 44. In other respects the separation of impurities and the flow of metal are similar to the method just described.

In the form of apparatus illustrated in Fig. 7, the duct 50 for the upward passage of gas and metal is connected at its lower end to the cover I I and extends a short distance into the pocket I. The upper end connects to the chamber 3 so as to discharge the metal and gas near the upper levell of metal therein. I connect the upper part of a special return duct 5i to the chamber near the piace where the metal is discharged from duct 50 and the lower part to the cover The special return duct may be quite small in diameter, for example, just large enough to induce a ow of metal in duct 58. That part of duct 50 which is above the upper surface of the bath is protected with thermal insulation I5 but the outsidesurface of duct 5| is left bare or at least provided with less insulation than duct 50 to induce a downward flow of metal in duct 5I.. The chamber 3 has one or more receptacles 4, a return duct 6 for the return of impurities to the bath and valve controlled passage I8.

In carrying out a method of the invention in the apparatus of Fig. 7, the metal and gas pass upwardly in duct 50 and a part of the metal is returned from the chamber to the bath by the special return duct 5|. This loop circulation of metal increases the rate of ilow of metal in duct 50, thereby preventing the accumulation of incrustations on the wall of the duct 50. The gas escapes through the valve I8 and the preliminary separation of impurities in pocket I, the flow of metal and separation of impurities therefrom in the chamber 3 and return of impurities to the bath in duct 6 are similar to the method described in connection with the apparatus of Figs. l to 4. In the apparatus o f Fig. 8, the upright duct 60 enters through the top of the usual metal collecting hood and is the means by which both metal and gas pass upward from the hood into the chamber 6|. As is the other forms of apparatus, the duct 60 is preferably provided with a thermal insulation 62 to prevent chilling where it extends above the ce11 cover. The chamber has a single hopper-shaped receptacle 63 for con'- iining the body of metal and passage 64 leading into the receiver 65. At the converging sides of the hopper, areturn duct 66 of relatively small cross-sectional area is attached and extends through the top of the hood and into the bath. I prefer to are the lower part of the return duct where it passes through the lower surface of the metal and to mount a baille 61 thereunder to keep gases from rising through the return duct. In this form of the invention, the return duct is constricted in diameter, that is, it has such small cross-sectional area that the metal therein oiers high resistance to the upward flow of heat into the body of metal. I may, instead of using a uniformly small duct as shown, use a larger duct and place a constriction in only a part of the duct.

The smaller the duct, or constriction in the duct, through which impurities will return to the bath without clogging the duct, the better. The constricted part of the duct shouldbe large enough only to permit the passage of impurities. I have found that a constriction around one inch in diameter may be used satisfactorily.

heat flowing upward in the metal in duct 66, I may place a thermal insulation 68 between the duct and the bath and leave annular spaces 69 and 'I0 through which I may circulate a cooling fluid to remove heat from the metal.

In the apparatus illustrated inFig, 9, the duct 'I2 for passing metal and gas upward into chamber 'I3 and the return duct 'I4 each pass through the top of the usual metal collecting hood, The duct 'I2 barely projects beneath the under side of the hood and accordingly practically elim-v inates the layer of gas such as is shown in Fig. 7, for example. The duct I2 has a thermal insulation 'I5 for the purposes hereindescribed and the hopper-shaped receptacles 'I8 connes the body of metal.

The return duct I4 is relatively large, being several inches in diameter, for examplefabout three inches. Where the duct connects to the bottom of the receptacle 'I6 and extends down to the. vicinity of the upper .surface of the bath, I mount a rotatable conveyor 82 which is suspended by rod 83 from the thrust bearing .84. The c'onveyor has a helical external thread or groove 85, and when it is rotated by means of the shaft 83, the metal and impurities accumulated in the bottom of the receptacle I6 are forced downward into the bath. Since duct 'I4 is relatively large, I may omit the ared bottom illustrated in other forms of the invention. In order, however, to prevent obstructions from accumulating, I may attach a rod 86 to the conveyor having a blade 8l which rotates with the conveyor. The conveyor 82 may be made of iron-which has a high resistance to the flow of heat and it, accordingly, serves as an effective thermal barrier suppressing the ow of heat up into the body of metal. The conveyor not only serves as a heat valve preventing the upward flow of heat, but as a mechanical valve checking the free flow of metal. In its main function of forcing the impurities to the bath, it

acts as a pump.' As in the other forms of the i'xvention, I prefer to place a baille 88 under duct The apparatus illustrated in Fig. 10 is similar to the apparatus of Fig. 9 with the exception that the return duct 90 is not provided with a conveyor. At a position near the cell cover, a thermal barrier 9| is mounted on a pivot 92 permitting it to be adjusted in the duct; The barrier is formed of metal and illled with an insulating material such as diatomaceous earth. It is shaped to serve the added function of a baille in directing the flow of impurities from the receptacle backA into the bath. I may also mount a baille 93. at the upper opening into the duct to direct the impurities over to one side of the duct where they ow through a narrow slit 94. By a suitable adjustment of thermal barrier 9|, I may induce a slight circulation of metal up along the right side of the duct as shown by the arrow. This circulation may be intensiiled by applying the insulation to the duct. In order to prevent gas released from thebath from vpassing upward in the duct 90, I mount the baille 96 in the bath.

As shown in Fig. l1, th`e metal and gas collected under the hood flow upwardly through duct |00 into the chamber |0I. The single hopper-shaped In Aorder to aid in maintaining the receptacle with the central opening thereof directly above the opening into the return duct. Convection currents from the return duct flow upwardly into baffle |04 and are returned, ai shown by the arrows. In order to facilitate this action, I may place a cooler in the baille to chill the metal and force it to ow in a downward direction. The cooler may be in the form of a conducting metal, such as a bar of copper which extends outside the cell, or a vessel |05 in which I circulate a cooling fluid through the tubes |03. The baiile |04 may be in the form of a single sheet of metal, or it may have double walls with a space therebetween filled with diatomaceous -earth or the like as shown in the drawings.

'I'he apparatus illustrated in Fig. 12 is generally similar to that of Fig. 11, with the exception of the return duct and its immediately associated parts. 'I'he return duct I |0 is relatively large and at th upper surface of the bath I arrange a thermal insulation comprising the space and jacket ||2 to diminish the flow of heat into the return duct. In the opening from the receptacle leading into the return duct, a thermally insulated baille |3 is suspended leaving a small clearance on the outside for the downward now of impurities and metal, as shown by the arrows. The baille I3 has a central tube ||4 which is flared at the bottom and constricted at the top. Immediately above the constriction at the top, a bellshaped baille ||5, like that described in Fig. 11, is suspended. In the central part of the baille I5, a depending adjustable. conical plug ||0 is arranged forV controlling the flow of electrolyte upwardly through the tube I4.

In operation, the apparatus illustrated in Fig. 12 has a bod-y of quiescent metal in the receptacle |02 and the thermal convection currents are prevented from indiscriminately flowing through the body of metal by means of a control over the direction of flow of the thermal convection currents originating below. The rising currents pass through tube I4 and up into thebaiiie H5 where the metal is cooled and by reason of an increase in density returns to the space between the return duct ||0 and the baille ||3. The thermal convection currents accordingly flow more or less in a loop and the downward flowing stream carries the impurities which separate out of the quiet body of metal downward into the bath.

I have found it advantageous to reduce the temperature of the body of metal in the chamber to a temperature very close to its fusion point in order to precipitate the dissolved impurities. The viscosity of the body of metal near its fusion point is relatively high making the settling of the precipitated sedimentary impurities slow. I, accordingly, provide any suitable means for reheating the body of metal, Just before it is discharged into the receptacle, sufficiently to decrease its viscosity and effect a more eiiicient settling of the impurities. To this end, I mount an' electric heating coil |20 just above the body of metal, as shown vin Fig, 13. The heating coil is preferably spaced near the place where the metal overflows into the,

receptacle so that the upper surface of metal is heated just before it flows into the receptacle. In order to prevent an undue flow of heat into other portions of the body of metal, I place a wall |2| across the upper part of the chamber. I may use other means of reheating the metal after it has been well cooled by means of coils or the like. g

Fig. 14 shows the application of my invention inthe production of metal by chemical reactions as, for example, the production of sodium by the reduction of sodium chloride by calcium 'I'he apparatus comprises a steel externally heated reaction vessel, a cover, a feed hopper |30 having an attached pipe |3I entering the bath, and a metal collecting hood submerged in the bath. When the reaction vessel is circular in horizontal cross-section, the hood is preferably annular and has a depending skirt |32, a duct |34 for the introduction of raw materials into the bath and for the removal of solid materials from the bottom of the bath.

At appropriate places in the top of the hood, the gas and metal duct |35 and return ducts |30 and |31 are connected. The metal collecting hood may be of the usual construction and the duct or ducts, as the case may be, for the upward passage of gas and metal is preferably arranged to permit separate layers of 'gas and metal to form under the4 hood. lThe metal may be permitted to accumulate to an extent which permits a preliminary separation by gravity of a large part of the entrapped salt and other sedimentary impurities before the metal passes upward into the chamber.

In practicing a methodv of the invention in operations where metal is formed by chemical reaction as, for example, in apparatus such as that 'A illustrated in Fig. l4,`the fused bath of mixed salts in the reaction vessel is fired externally as by oil or gas flames. One reagent, for example,

calcium, is added through the hopper |30. Thepurities from the metal in the chamber is similar to that previously described.

In precipitating dissolved impurities and in separating and removing the sedimentary impurities from the body of metal in accordance with the various adaptations of the invention herein V described, it is important to maintain certain temperature conditions in the metal.

'I'he crude hot metal (sodium) collected in the bath is usually at a temperature of about 600 to 700 C. I prefer that this metal .flow into the chamber without appreciable cooling; in other words, I prefer to pass it into' the chamber at the highest practicable temperature. The metal entering the chamber is cooled several 4hundred degrees and in its passage through the chamber the temperature of the metal is reduced to within a few degrees of its fusion point. In the case of sodium, I may drop the temperature of the metal to as low as C. By coolingthe metal in the chamber to a temperature close to the melting point of the metal, I am able to effect a high degree of puriiication and may eliminate entirely the necessity of additional refining of the' metal. Before discharging themetal into the receiver, I

may increase the temperature to decrease the viscosity and further facilitate settlingof the impurities. i

In the apparatus of Figs. 1 to 7, I may eliminate the pocket and connect the ducts directly t0 the usual metal collecting hood. as shown in c Figs. 8 to 14, and I may use any of the'various return ducts of Figs. 8 to 13 in place of the return ducts 6 and 1. I may so arrange any of the ducts through which the metal and gas pass upward with respect to the top of the pocket or the top of the hood that the layer of gas may be of any desired thickness or even eliminated. The apparatus of Figs. 5, 7 and 8 to 14 is understood to be provided with a housing similar to housing 2@ of Figs. 1 to 4, and instead of connecting the ducts directly to the metal collecting hood, I may connect them to a pocket, as shown in Figs. 1 to 4, for the rpreliminary puriilcation of the metal. The chambers of Figs. 1 to "I, 13 and 14 are divided into a plurality of connected receptacles and I may use in place of these a single receptacle as shown in Figs. 8 to 12. I may also use any suitable arrangement of baiiles in the chambers, for example, as shown in' Fig. 4, to effect a control over the settling of the impurities. I may apply the heating of the body of metal as shown in Fig. 13 to any of the other forms of the invention.

I claim:

1. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which .comprises collecting crude hot metal in the bath, passing the crude hot metal upward in a column of metal resting on the bath in'to a body of metal above the bath, cooling the crude hot metal in the body of metal to a point where dissolved impurities are precipitated, maintaining the metal in the body in such a quiet state that the precipitated impurities settle, and returning the precipitated impurities to the bath through another column of metal which connects the body of metal with the bath. v

2. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises -collecting crude hot metal in the bath, passing the crude hot metal upward in a stream into a body of metal above the bath, cooling the crude hot metal in the body of metal to a point where dissolved impurities are precipitated, maintaining the metal in the body in such a quiet state that the precipitated impurities settle, and forcing some metal. and the precipitated impurities back into the bath.

3. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing the crude hot metal upward in a column of metal resting on the bath into a body of metal above the bath, cooling the crude hot metal in the body of metal to a point where dissolved impurities are precipitated, maintaining the metal in the body in such .a quiet state that the precipitated impurities settle, and returning the precipitated impurities t the bath through anotherv column of metal which Y. connects the body of metal with the bath and which is free of convection currents interfering with the return of impurities to the bath.

4. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing the crude hot metal upward in a stream into a body of metal above the bath, cooling the crude hot metal in the body of metal to a point where dissolved impurities are precipitated, maintaining the metal in the body in such a quiet state that the precipitated impurities settle, maintaining a column of metal in contact with the body of'metal and the bath, suppressing the flow of heat from the bath through the column into the body of metal, and

-settling the impurities through the column of metal to the bath.

5. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing thecrude hot metal upward in a stream into a body of metal above the bath, cooling the crude hot, metal in the body of metal to a temperature just above the melting point of the metal to precipitate dissolved impurities, removing the impurities from th'e metal by settling, and returning the impurities to the bath through a column of metal out of contact with the rising stream of metal.

6. In the method of claim 5, cooling the crude hot metal to a temperature less than iifteen degrees above the melting point of the metal.

'7. In the method of claim 1 passing the hot crude metalinto the body of metal near its upper surface and flowing it over cooler metal in the body to reduce very materially the temperature of the hot crude metal, thereby precipitating the dissolved impurities.

8. In the method of claim l passing the crude hot metal into the body of metal near its upper surface and flowing it over cooler metal in the body to reduce very materially the temperature of the crude hot metal, thereby precipitating the dissolved impurities, and heating the metal to decrease its viscosity and facilitate settling of the impurities.

9. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing the crude hot metal upward in a stream into a body of metal above the bath, cooling the crude hot metal in the body of metal to a point where dissolved impurities are precipitated, heating the cooled metal to decrease its viscosity suiiiciently to facilitate- ,settling of the impurities, and removing puried metal from the body.

10. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crudehot metal in the bath, passing the hot metal upward in a stream into a body of metal above the bath, dividing the metal in the body between a plurality of connected receptacles, passing the metal from receptacle to receptacle, cooling the crude hot metal progressively from receptacle to receptacle in the direction of metal flow, settling impurities to the bottom of each receptacle, and returning the impurities to the bath from each receptacle through a separate column of metal.

11. In the recovery of iight metal from a fused salt bath'more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing the crude hot metal upward in a column of metal resting on the bath into a body of metal above the bath, flowing the crude hot metal through the body of metal and cooling it progressively in the direction of metal flow to precipitate dissolved impurities, settling the precipitated impurities, and returning the precipitated impurities to the bath in another column of metal connecting the body oi metal with the bath.

12. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal in the bath, passing the crude hot metalupward in a stream into a body of metal above the bath, flowing the crude hot metal throughthe body of metal and cooling it progressively in the direction of metal flow to precipitate dissolved impurities, heating the metal slightly after the cooling has been completed to decrease the viscosity and facilitate settling of the precipitated impurities, and returning the impurities to the bath.

13. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method `which comprises collectingcrude hot metal in the bath, passing the hot metal upward in a stream into a body of metal above the bath, maintaining the body of metal in such thermal isolation from the bath that a large portion of the metal in the body is at a temperature less than half the temperature of the bath, precipitating impurities dissolved in -the crude hot metal, and returning the impurities to the bath by settling them through a column of metal connecting the body with the bath.

14. In the method of claim 13 which comprises providing a column of metal for theV return of impurities in which the flow of heat to the bodyC of metal is suppressed.

15. In themethod of claim 13 which comprises confining the body of metal in a chamber spaced.

from the bath, said body of metal being connected to the bath only by the rising stream of metal and assaesa of heat from the bath into the metal in the return duct.

23. Apparatus according to claim 22 which comprises means in the return duct for forcing metal and impurities intb the bath.

24. Apparatus according to claim 22 which comprises a constriction in the return duct to suppress the flow of heat through the metal the column of metal, and suppressing the llow of heat through the column into the body of metal.

16. In the recovery of light metal from 'a fused salt bath more dense than the metal recovered,

dissolved impurities from the crude hot meta1,

settling the impurities, and returning the impurities in a column of metal in another duct connecting the chamber with the bath.

17. In the method of claim 16 suppressing the ilow of heat through the column of metal into the body of metal.

18. In the method of claim 16 forcing the metal in the column of metal to flow in the direction of the bath. I

19. In the method of vclaim 16 which comprises removing gas collected with the crude hot metal under the bath through the first-mentioned duct.

20. In the method of claim 16 which comprises removing gas collected under the bath through a third duct.

21. In the recovery of light metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude hot metal and gas beneath the upper surface Iof the bath, passing the crude hot metal in a stream to a body of metal above the bath, precipitating impurities from the crude hot metal and returning the impurities to the bath, kecping the gas out of contact with the returning impurities, and discharging the gas to the atmosphere without bubbling it through they body of metal.

22. Apparatus for recovering a light metal from a fused salt bath more dense than the metal which comprises means for collecting the metal beneath the upper surface of the bath, a chamber above and spaced from the bath, a duct for passing metal collected beneath the bath into the chamber, a return duct connecting the bottom of the chamber with the bath for confining a column of metal in which impurities are returned to the bath., and means' for suppressing the now therein, and means for suppressing the flow of heat from the bath through the duct into the metal in the duct.

25. Apparatus according to claim 22 which comprises a bell-shaped baule ln the chamber above the return duct for ldiverting convection currents from the body of metal undergoing purification in the chamber.

26. Apparatus according to claim 22 which comprisesheating means for reheating metal in the chamber after it has been cooled.

2 7. In the recovery of metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude liquid metal in a quiet pool below the upper surface of the bath, removing sedimentary impurities from the crude liquid metal by passing the collected metal in a lateral direction through said pool Ibelow the upper surface of the bath, passing the metal from which impurities were removed upwardly under the action of gravity in a4 stream to a rbody of metal above the upper surface of the bath, cooling the metal in the `body to precipitate dissolved impurities from the metal, and returning the precipitated impurities to thex bath through a column of metal connecting the body of metal with the bath, said column of metal being spaced from the upward stream.

28,. In the recovery of metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude liquid metal and gas in the same place below the upper surface of the bath, allowing the collected metal to accumulate in the bath in a Y layer of appreciable depth, settling sedimentary impurities out of the metal in the layer and returning the impurities to the bath, separating the gas from the collected metal, passing the collected metal upwardly from the layer under the action of gravity in a duct to a body of metal above the level of the layer of collected metal and controlling the temperature of the duct and the rate of flow of metal passing therethrough to suppress the formation of incrustations on the duct, and settling impurities from the body of metal to the bath through a 'column of metal in another duct spaced to one side of the rst-mentioned duct.

29. In'the recovery of metal from a fused salt bath more dense than the metal recovered, the improved method which comprises collecting crude liquid metal and gas in the same place below the upper surface of the bath, allowing the collected metal to accumulate in the bath in a layer of appreciable depth, maintaining the layer of metal in such a quiet state that sedimentary impurities may settle out and return to the bath, separating the gas from the collected metal and discharging it to the atmosphere, passing the collected metal upwardly under the action of gravity through a column of metal resting on the bath to a body of metal which is entirely above the bath, cooling the metal in the body of metal to precipitate dissolved impurities, and returning the precipitated impurities to the bath in another column of metal connecting the-body of metal with the bath, said column of metal being spaced to one side of the mst-mentioned column, saidl body of metal being out of contact with the bath except by means ofthe said columns.

30. A separating device for the recovery ox light metals from fused salt baths which comprises means for collecting molten metal in a pool in the bath, said means for collecting molten metal being constructed to maintain said pool in avrelatively quiet state wherein a preliminary separation of impurities takes place, a metal receiving chamber spaced entirely .above the upper surface of the bath, an upright ducty for passing the collected molten metal from the pool into the chamber, means for cooling the metal in the chamber to effect a separation of sedimentary impurities from the metal, and a second duct spaced from the upright duct connecting the chamber with the bath for confining a column of metal resting on the bath and extending into the chamber, said column of metal in the second duct serving as a medium for the return of said impurities to the bath.

31.y Apparatus according to claim' 30 which comprises means for preventing undue cooling of the molten metal in the upright duct.

32. 'I'he combination with an electrolytic cell for the electrolysis of -vfused salt baths which comprises means for collecting crude molten metal below the upper surface of the fused salt bath, a chamber entirely above and spaced from the said surface of the bath for receiving the crude molten metal, one or more separating vessels in the chamber, an upright duct arranged to pass the crude molten metal collected below the upper surface of the bath into a separating vessel of the chamber, means for cooling the crude metal in the separating vessel to precipitatel dissolved impurities, means permitting the settling of the precipitated impurities from the crude metal in the separating vessel, means for returni118 the precipitated impurities to the bath at a. distance from and out of contact with the metal in the upright duct, and means for removing purified molten metal from a separating vessel.

33. The combination with an electrolytic cellA for the electrolysis of fused salt baths which ling at least one separating vessel, an upright duct connected to the outlet of the device and to the chamber arranged to pass the crude molten metal collected in the device into a separating vessel of the chamber, a second duct connecting the separating vessel with the bath, means for dlrecting sedimentary impurities separated from the metal in the separating vessel into the second duct through which the impurities may settle to the bath, and means for removing purine molten metal from the separating vessel.

34. Apparatus according to claim 33 which comprises a plurality of separating vessels interconnected for the flow of metal progressively from the upright duct across the vessels, duct means for the return of sedimentary impurities from each separating vessel to the bath, and means for removing purified molten metal from the sepa' rating vessel most remote from the upright duct.

ROBERT J. McNrr'r. 

